In certain types of mechanical seal, so-called barrier or buffer fluids are used to contain the process fluid. The barrier or buffer fluid is managed within a seal support system including a vessel or tank containing a volume of the fluid, generally water. The vessel is connected to the sealing device which may be situated in a pump, mixer or other item of rotating equipment. Normally a return pipe feeds water back to the vessel from the sealing device, thereby forming a “loop” whereby the water, contained in the vessel, may enter and exit the sealing device. The purpose of the barrier fluid, whether water or some other fluid, is to lubricate and cool the components within the sealing device, while at the same time being compatible with the process fluid.
Generally the sealing device on the item of rotating equipment is a mechanical seal having a rotating member secured to a shaft and a stationary member which is secured to a housing. The interface, between the rotating member and the stationary member, prevents the process fluid from escaping. Most mechanical seals have a fluid film, acting between the two sliding seal faces. Thus fluid film lubricates the seal faces but there will be a tendency for the fluid to leak, the amount of leakage depending on the gap between the two sliding faces. Furthermore, pressure surges and sudden rises in fluid temperature can result in barrier fluid loss. In addition further loss of fluid from the vessel may be due to evaporation.
GB-A-2381838 discloses a water management system, for use with a mechanical seal, which ensures that, if the seal fails, the water flow into the system is stopped, thereby minimizing product contamination or “watering down”.
This system functions well except when a particular situation may arise. If the faces of a mechanical seal are forced apart, as a result of a process upset, a significantly increased barrier fluid flow takes place. This excessive flow is detected and the device operates so as not to allow any more barrier fluid to enter the system. If the faces of a mechanical seal then return to their normal working positions, and there is no damage to the mechanical seal, the seal will continue to run without any further barrier fluid entering the system. As a result the seal will run dry and fail.